As mobile data service volume continues to grow, spectrum resources has become more and more scarce, and for network deployment and service transmission, it is impossible to meet the requirements of the increased data service volume by merely using licensed spectrum resources. Hence, for a Long Term Evolution (LTE) system, the network development and the service transmission on unlicensed spectrum resources may be taken into consideration so as to improve the user experience and expand a network coverage, and this LTE system may be called as Unlicensed LTE (U-LTE or LTE-U for short). However, currently, there is no concrete scheme about how the LTE system can operate on the unlicensed spectrum resources.
A Listen Before Talk (LBT) principle on the unlicensed spectrum resources will be described as follows.
For the unlicensed spectrum resources, no specific application system has been planned, and they may be shared by various radio communication systems, such as Bluetooth and Wireless Fidelity (WiFi). The shared unlicensed spectrum resources may be used by various systems through resource preemption. Hence, coexistence between LTE-Us deployed by different operators or between the LTE-U and the radio communication system such as WiFi has become a research emphasis and difficulty. As required by the 3rd Generation Partnership Project (3GPP), the fair coexistence needs to be ensured between the LTE-U and the radio communication system such as WiFi, and a main carrier of a licensed frequency band may aid the implementation of an unlicensed frequency band as an auxiliary carrier. As a basic method for the LTE-U contention access, the LBT has accepted by almost all the companies.
The essence of the LBT technology is still a Carrier Sense Multiple Access/Conflict Avoidance (CSMA/CA) mechanism adopted by a 802.11 system. The WiFi system may preempt the resources on the unlicensed frequency band in the following ways. At first, it may monitor a channel, and in the case that an idle time of the channel reaches a Distributed Inter-Frame Space (DIFS), it may determine that the current channel is a clear channel. Then, sites which are waiting for access to the channel may be in a random backoff stage, so as to prevent the collision of the sites on the same resource. In addition, in order to ensure fairness, it is further speculated that the spectrum resource cannot be occupied by each site for a long time period. As a certain time period expires or a data transmission volume reaches an upper limit, the resource needs to be released, so that the other WiFi or LTE system can preempt the resource.
In the case that the LTE system operates on a carrier of the unlicensed frequency band, an LTE base station and a User Equipment (UE) also need to use the LBT mechanism for resource contention, so as to ensure that the spectrum resources can be shared fairly between the LTE system and any other device or system.
According to an European Standard, two methods for the LBT mechanism on the unlicensed frequency band will be described hereinafter.
As specified by European Telecommunication Standards Institute (ETSI), there are two modes for the LBT mechanism on the unlicensed 5 GHz frequency band, i.e., a Frame-based Equipment mode and a Load-based Equipment mode.
Referring to FIG. 1, in the Frame-based Equipment mode, a fixed frame occupation duration is provided, including a data transmission duration and an idle time period which is not smaller than 5% of the data transmission duration. Following a fixed frame, a Clear Channel Assessment (CCA) detection time period (at least 20 us) for detecting whether or not the channel is a clear channel may be provided. The channel may be used only in the case that an unlicensed device determines within the CCA detection time period, in an energy detection manner, that the channel is clear. For example, the unlicensed device may measure power of a signal received over the channel within the CCA time period, and in the case that received signal power over the channel is greater than a first power threshold, it may determine that the channel is busy; otherwise, it may determine that the channel is clear.
Referring to FIG. 2, in the Load-based Equipment mode, the data transmission duration is variable. Before the data transmission over an unlicensed channel, the unlicensed device needs to perform, in an energy detection manner, CCA detection on the channel once. In the case that the channel is clear, the unlicensed device may transmit data over the channel, and in the case that the channel is busy, the unlicensed device may detect the channel in a CCA-extended manner. During the CCA-extended detection, the unlicensed device needs to detect N CCA time periods within which the channel is clear, so as to determine that the channel is clear, and then, the unlicensed device may transmit the data over the channel. N is of a random value within the range of 1 to q, and q is of a value within the range of [4, 32].
Currently, there is no concrete scheme about how the LTE system can operate on the unlicensed frequency band. Also, for an unlicensed carrier, there is still no concrete scheme about the resource contention between the LTE base stations or UEs belonging to an identical operator.
Based on the LBT mechanism for the resource contention on the unlicensed carrier, in the case that the LTE system operates on the unlicensed carrier and each LTE base station/cell/UE generates its own CCA detection time period separately and participates, as an individual entity, in the resource contention on the unlicensed carrier, there may exist the following problems.
For a network side device, e.g., each LTE base station or cell of the operator, it may determine, through the following two steps, whether or not it can occupy the channel through contention after its own CCA detection time period expires.
Step 1: at first, the network side device needs to detect within a CCA detection window whether or not a signal has been transmitted (power detection), and in the case that the channel is clear (e.g., the power detected over the channel is smaller than a predetermined threshold), it may directly occupy the channel.
Step 2: in the case that there is signal transmitted over the channel, the network side device needs to further determine whether or not the signal transmitted over the channel is an LTE signal and whether or not this signal belongs to an identical operator. In the case that the signal belongs to the same operator, it may still occupy the channel (because the LTE system may support co-frequency networking). In the case that the signal belongs to a different operator, it may abandon the occupation of the channel. At this time, the network side device needs to detect or parse the signal (e.g., detect a Cell-specific Reference Signal (CRS) so as to acquire a cell identity (ID) and determine the operator to which the signal belongs). In order to detect and parse the signal, a certain time delay needs to be provided, e.g., at least a series of CRSs need to be detected, resulting in certain resource overhead and an increase in the processing complexity of the base station. In addition, due to signal interference between different base stations, it is impossible to ensure the detection performance, and thereby the LBT performance may be adversely affected.
At a terminal side, according to the conventional LTE mechanism, there is a fixed timing relationship between a Physical Downlink Control Channel (PDCCH) and an Uplink-Granted Physical Uplink Shared Channel (PUSCH) of the LTE system, and usually the PDCCH may precede the PUSCH by K subframes (K>=4). However, based on the contention mechanism on the unlicensed carrier, the UE needs to perform uplink channel resource contention before the transmission of the PUSCH, and it means that the base station may not determine, in the case that the base station schedules the PDCCH for the UE, whether or not the UE can acquire, during the transmission of the PUSCH, a UL resource after K subframes through contention. For example, the scheduled UE needs to occupy a channel through contention with a device of the other unlicensed system, and occupy a UL channel through contention with the other LTE UE (within an identical cell or neighboring cell) on an identical carrier. Hence, it is impossible to support multi-user scheduling or Multi-User Multi-Input Multi-Output (MU-MIMO) features. Due to the above-mentioned problems, the system efficiency may be degraded significantly.
In a word, currently there is no solution about the channel resource contention for the LTE system in the case that it operates on the unlicensed carrier.